A typical wireless communication system operating according to the Evolved Universal Terrestrial Radio Access (E-UTRA) standards includes a plurality of base stations referred to as evolved Node B's (eNodeBs), each providing radio frequency (RF) coverage defining a cell in which to serve user equipment devices (UEs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped communication devices, whether or not technically user operated. The eNodeBs further sit as nodes on a core packet-switched network, though which the eNodeBs interface with a network controller referred to as a mobility management entity (MME) and with a system of gateways, including serving gateways (SGWs) and packet-data network gateways (PGWs) that provide connectivity with one or more transport networks such as the public Internet for instance.
Each cell in such a system typically operates on a carrier frequency and defines various air interface channels for carrying information between the eNodeB and UEs.
On the downlink, for instance, each cell may define synchronization channels for carrying signals that UEs can detect as an indication of coverage and that indicate a physical cell identifier (PCI) of the cell. Further, each cell may define a cell-specific reference channel for carrying a reference signal that UEs can measure to evaluate coverage strength, a downlink control channel for carrying system information, page messages, and other control signaling from the eNodeBs to UEs, and a shared traffic channel on which the eNodeB can allocate resources for carrying bearer traffic from the eNodeB to UEs.
And on the uplink, each cell may define an access channel for carrying random access signaling from UEs, an uplink control channel for carrying service requests, measurement reports, and other control signaling from UEs, and an uplink shared traffic channel on which the eNodeB may allocate resources as needed to carry bearer traffic from UEs.
When a UE initially enters into coverage of such a system (e.g., powers on in coverage of the system), the UE may scan the airwaves in search of a cell providing a sufficiently strong reference signal, and the UE may then work to connect with the eNodeB providing that signal and to register for service with the network. For instance, the UE may engage in random access signaling with the eNodeB and then transmit to the eNodeB a Radio Resource Configuration (RRC) request message, and the eNodeB may grant the request, establishing an RRC connection defining a radio-link-layer connection between the UE and the eNodeB. Here, the eNodeB may assign to the UE an RRC connection identifier (C-RNTI) and establish a UE context record noting the existence of the RRC connection. Further, the UE may transmit an attach request to the eNodeB, which the eNodeB may forward to the MME over an S1-MME interface, to register the UE for service with the network.
Upon receipt of the UE's attach request, the MME may engage in a process to authenticate the UE. And the MME may then coordinate establishment for the UE of one or more bearers defining logical channels through the core network for carrying bearer traffic to and from the UE via the eNodeB. In particular, the MME may engage in signaling with the eNodeB and with a serving gateway (SGW) to establish for the UE an S1-U tunnel between eNodeB and SGW, the SGW may responsively engage in signaling with a packet-data network gateway (PGW) to establish an S5 tunnel between the SGW and the PGW and may bridge the S5 tunnel with the S1 tunnel, and the PGW may assign an Internet Protocol (IP) address useable by the UE to engage in packet-data communication.
Once the UE is thus RRC connected with the eNodeB and has one or more such bearers extending via the eNodeB, the eNodeB may then serve the UE with wireless data communications.
For instance, when the UE has packet-data to transmit, the UE could transmit a scheduling request to the eNodeB, the eNodeB could allocate uplink shared channel resources for use by the UE, and the UE could transmit the data on those resources. Upon receipt of the data, the eNodeB could then forward the data via one of the UE's one or more bearers to the SGW, the SGW could forward the data over the bearer to the PGW, and the PGW could output the data for transmission to its destination. Likewise, when data arrives at the PGW for transmission to the UE, the data could flow over one of the UE's one or more bearers via the SGW to the eNodeB, the eNodeB could allocate downlink shared channel resources for the transmission, and the eNodeB could transmit the data on the allocated resources to the UE.
In addition, while the UE is RRC connected, the UE may also work to ensure that the UE continues to be served by a sufficiently strong coverage area, and perhaps by the strongest available coverage area.
To facilitate this, the UE may continue to regularly monitor reference signal strength from its serving eNodeB and, perhaps upon detecting that its serving eNodeB's reference signal strength becomes threshold low, may scan the airwaves in search of another cell with threshold strong coverage. Upon detecting such another cell, the UE may then transmit to its serving eNodeB a measurement report that specifies the PCI and signal strength of the detected cell. And in response to the measurement report, the eNodeB may then engage in a process to trigger handover of the UE from being served by the eNodeB to being served instead by the eNodeB that provides the reported cell.